Recently, with the development of office automation, copying machines, printers, facsimiles, etc. utilizing various recording systems such as an electrophotographic system and a thermal transfer recording system have been widely used. These recording systems are used also in CAD/CAM, etc. for example according to the purposes thereof. In this case, colored color materials are used for forming images. Usually these color materials are transferred to a recording medium such as a paper and a film sheet by melting, evaporating or sublimating said color materials, a recorded image being obtained by adhering, absorbing and dyeing actions.
Among these recording systems, attention has recently been paid to a thermal transfer recording system of a heat melting type in which an ink ribbon having a thermal-meltable ink layer comprising color materials is melted by the heat of a thermal head, said color materials being transferred to a recording sheet, a recorded image being obtained by adhering, absorbing and dyeing actions. This recording system has a characteristic feature that it is possible for use an plain paper (wood free paper) as a recording medium.
In said thermal transfer recording system, as in other recording systems, there are increasing demands for full-color recording, high-speed recording, clear images, high resolution, etc. In single-color recording or multi-color recording by a color-thermal transfer printer, an ink ribbon having color materials such as yellow, magenta, cyanogen and black as well as waxes and resins is combined with a recording sheet, a transfer image being formed on said recording sheet by means of a thermal head. Since inks of various colors lie one above the other, said thermal transfer recording system has the disadvantages that unevenness of image and loss of dots (ink) are liable to occur owing to the improper smoothness of the surface of the image receiving layer.
Various proposals have been made to improve the smoothness of the surface of the image receiving layer by coating or saturating a substrate with a coating composition comprising pigments and binders instead of using a plain paper as it is. These proposals include inventions specifying a Bekk smoothness (Japanese Patent Laid-Open Publication No. Sho 59-133092 and Japanese Patent Laid-Open Publication No. Sho 59-187892) and inventions providing a heat transfer image layer comprising specific pigments and binders (Japanese Patent Laid-Open Publication No. Sho 57-182487, Japanese Patent Laid-Open Publication No. Sho 59-182787, U.S. Pat. No. 4,639,751, Japanese Patent Laid-Open Publication No. Sho 60-11489, Japanese Patent Laid-Open Publication No. Sho 60-110492, Japanese Patent Laid-Open Publication No. Sho 60-192690, Japanese Patent Laid-Open Publication No. Sho 61-217289, Japanese Patent Laid-Open Publication No. Sho 61-286187, Japanese Patent Laid-Open Publication No. Sho 63-21185 and Japanese Patent Laid-Open Publication No. Hei 1-253478). Also, an image-receiving sheet comprising a non-coated plain paper using a specific paper-making filler is disclosed by Japanese Patent Laid-Open Publication No. Sho 61-225396, Japanese Patent Laid-Open Publication No. Sho 63-19289, etc. The prior art described above has some improvements but does not completely prevent unevenness of image or color difference at portions where color inks lie one above the other in multi-color recording, or the reduction of image clearness owing to the loss of dots or to the improper reproduction of dot shapes.
The inventors consider that it is insufficient to improve smoothness by strengthening calendering, etc. or to make a thermal transfer receiving layer contain specific pigments or binders. No practicable art has been developed so far which obviates all the disadvantages of the prior art and ensures an image-receiving sheet for a thermal transfer recording system, said image-receiving sheet ensuring excellent ink transfer and dot reproduction.
Recently, image-receiving sheets for a thermal transfer recording system are often subjected to printing. This situation requires that the image-receiving sheets have a suitable smoothness, surface strength, opacity, etc. Furthermore, paper dust produced in cutting the image-receiving sheets affects the working environment of the users. Such a trouble must be immediately remedied.